Recovery of lower acid anhydrides by distillation



Dec- 22, 1953 D. c. HULL HAL 2,563,6s

RECOVERY 0F' LOWER ACID ANHYDRIDES BY DISTILLATION Filed July 2l, 1950 mason z8 L WILLIAM R. SAUNDERS :inventors Bu Md N,

Cttornegs Patented Dec. 22, 1953 UNITED STATES ATENT OFFICE RECOVERY OF LOWER ACID ANHYDRIDES BY DISTILLATION Jersey Application July 21, 1950,7seria1-No. 175,268

2 Claims. Cl.V 202-4-40) This invention relates to the separation and concentration of anhydrides of organic acids. More particularly the invention is concerned with the process and apparatus fcrthe concentration and separation of acetic anhydride from crude solutions thereof wherein the acetic anhydride is in the presence of acetic acid, Water, and the like.

.es is known, anhydrides of organic acids may be manufactured by several methods such as the pyrolysis of the corresponding acid, the oxidation of aldehydes, or the reaction of Various chemical derivatives with other components to obtain a crude anhydride composition. This crude or raw anhydride as produced may contain varying amounts of acid, Water, and other components. For example, in the manufacture of acetic anhydride by procedure linvolving the pyrolysis of acetic acid the resultant crude anhydride may contain substantial amountsof acetic acid andwater.

inasmuch as anhydrides are of reactive nature exhibit a strong afnnity for water, there'is a problem in the efficient and economical separa-y tion of the anhydride in order to isolate a reasonably pure anhydride without too great a loss.

A number of methods have already been proposed In other processes, such as described in Patent 2,166,997, a spray arrangement is proposed for injecting the anhydride mixture into heated acid for vaporizing away water which is present. Such prior types of processes, such as for example the introduction of an organic entraining agent, may leave small residual amounts of the entraining agent in the anhydride nally isolated, which small contamination mightbe objectionable should the anhydride be desired for The utilization of spe-- certain exacting uses. cia-lined mechanical equipment in the processing of anhydride presents problems because of the corrosive nature of the organic acids.

It has also been proposed to isolate anhydride by the simple fractionation thereof in a single fractionating column, operated either under normal, or part of the time, under reduced pressure. Such direct fractionation in a single column permits of isolation of the anhydride without contamination by an entraining agent. However, to obtain reasonably complete fractionation in accordance with such prior practice of utilizing a single column, reflux ratios of the order of 8 to 1 were employed. It is, therefore, apparent that considerable heat input is required in operating such a single column and limited capacityprevails because a substantial part of the output must be reutilized as reflux in the single column fractionation of the prior art.

It has now been found, in accordance with the present invention, how anhydride of as good a quality as obtainable by the single column fractionation, aforementioned, may bel isolated in a more economical and efficient manner. That is, it has been found that by means of a certain combination of columns, such as for example with a combination of two columns of slightly increased length, that considerably more than 'twice' the anhydride output of a single column may be obtained at less than twice the heat in'- put now required for single column operation.

This invention has for one object to provide a process and apparatus arrangement for theV separation of anhydrides of organic acids. Another and more specific object is to provide-a process and apparatus arrangement particularly adapted to the separation and isolation of acetic anhydride from solutions, liquids, crude admirtures, or the like containing the anhydride in the presence of acetic acid, water, or other similar low boiling components. A still further object is to provide a process of the class described which is more eiiicient and economical in that it requires less heat input yet is capable `of delivering larger amounts of anhydride. Still another object is to provide a novel combination of fractionating units particularly adaptable to the processing of anhydride containing liquids. Other objects will appear hereinafter.

For a more complete understanding of the present invention, reference is made to the attached drawing forming a part of the application. This drawing comprises va semidiagrammatic side elevation view somewhat in the nature of a flow sheet illustrating an apparatus arrangement in accordance with the present invention.

Referring now to the drawing, there are shown two rectifying columns designated 2 and 3. These columns, as for example column 2, may

comprise a standard bubble plate fractionating column of conventional type. That is,' the standard singie column discussed above already in use in an anhydride plant may be employed. Ereferably the column will contain the equivalent oi at least 2G plates. However, if it is a shorter column, accordance with the present invention it is preferred to add a few additional plates to the top of the column, thereby increas ing its height slightly so that there is available a column having the equivalent of 2G to 30 plates, for example. lt is to be noted that the exact number oi plates is not a limitation on the present invention, but for best operation there should be adequate plate capacity in the columns used.

With further reference to column 2 there is provided the usual vapor outa-ke conduit A which leads through a conventional condenser S. Any usual condenser may be employed, either refrigerated or water cooled. Hence, detailed description is not requiredf` In general, as it will be noted as the description proceeds, the individual parts making up the apparatus arrangement of the present invention may comprise standard commercially obtainable units.

Condenser G is provided with conduit i for withdrawal or" condensate. Redux line 8 is provided so that a suilicient portion of the condensate, as will be discussedv in detail, may be returned to the column for refiuxing.

Column 2 is also provided with a feed line designated S-i i. lt will be noted that this feed line extents through (by means oi coil i9) heat exchange device i2, as will be considered in greater detail hereinafter.

The lower portion of column 2 is suitably equipped with a base heater dagrammatically indicated at ld. This base heater is preferably of a special construction andconnected by a plurality of conduits to column i so that there can be adequate and emcient circulation of the cooled liquid from the bottom of the column to the heater and return of hot liquid. The base heater Ill is also provided with a vapor oiftake conduit l whereby vapors therefrom may be carried through the heat transfer device l2 already mentioned.

Referring further to base heater I4, this unit is connected to the bottom of column 2 by means of conduit Si?. This conduit conducts the liquid from column 2 into the base heater whereby heat may be supplied thereto. Column 2 is also connected to base heater id by means of conduit 3i which is attached to the upper portion of the base heater whereby the heated vapors from the base heater reenter the column. Base heater ld may comprise an enclosed tank of any desired coniiguration having positioned intermediate thereof a tube bundle 32 of usual construction. That is, the tube bundle is made up of tube sheets 33 and 3d which support the ends of a plurality of tubes illustrated by 35. The liquid to be heated passes up through these tubes. The outsides of the tubes are surrounded by steam or other heating medium supplied at 3e and withdrawn at 31.

Also attached to the base heater is a downcomer leg 33 attached at the top by conduit 39 and at the bottom by conduit l. This unit 38 is merely an enclosed vertically extending tank and has the following functions: It provides an entrainment separator which allows the disen gagement of liquid from vapor. It comprises a means for allowing convective forces to function, thereby helping to maximize the velocity of the liquid through tube bundle 32. This permits a high rate of heat transfer between the heat being supplied to tube bundle 32 Iandthe anhydride being heated therein. This construction is also advantageous in that it prevents the anhydride sludge or the like components which may be present in the anhydride undergoing distillation from fouling the heating elements of the base heater.

Referring further to down-comer 38, there is provided from the top thereof the conduit i6 whereby vaporized anhydride and acid may be conducted to the heat exchanger as already described. It is mentioned at this point that the details of the construction of base heater 23, to be referred to hereinafter, are the same as base heater M. Therefore, a detailed description under 23 will not be necessary.

Column 3 may be of the same general construction as column 2, that is, the columns may be of equal size and have approximately the same number of plates, but this is not essential as columns of unequal size may be employed. In the latter instance it is preferred that the larger column be used as column designated No. 2 in the present description. In further detail, column 3 is provided with a vapor oitalre line i?, one or more condensers i3, reflux line i9, and drawo line 2l.

The feed line to column 3 which is line 22 interconnects the units by virtue of its attachments to unit i2 in a manner so that the liquid collected in i2 may flow therefrom to column 3. While it is possible to pump the liquids, for simplicity of construction, it is preferred that the arrangement and positioning of the parts be such that advantage may be taken of gravity flow, thereby avoiding the necessity of pumping a hot acid-containing liquid.

Column 3 is also provided with a base heater 23 which is connected by conduit 2d to a suitable anhydride condenser 25 from which anhydride may be withdrawn through conduit 28.

Referring now in further detail to the heat transfer unit i2, this unit may be a standard com.- rnercially available heat exchanger of a construction adapted to withstand organic acids and anhydrides. As may be observed from the drawing, unit l2 is somewhat similar in construction to a condenser in that the cold liquid entering through 9 picks up heat at lil from the hot vapors supplied through I. The liquid condensed within I2 flows out through conduit 22.

While a number of the parts may be constructed of copper or other commercial constructional materials, it is preferred to utilize stainless steel for many of the Darts such, as for example, the well known Cr-Ni-Mo type of stainless steel. On certain of the parts, such as for example, the condensers, it may even be desirable to use parts made of or plated with noble metals such as silver or silver plated final condensers for preventing any possible contamination of the anhydride nnally isolated.

The operation of the apparatus is apparent to a substantial extent from the drawing and the above description. However, further details rcspecting the process will be apparent from the following example concerning the separation of acetic anhydride in the presence of acetic acid and Water. The particular mixture separated in' accordance with this example comprised the con* densed crude anhydride as obtained from a pyrolysis process for the manufacture of acetic anhydride. This crude anhydride was comprised of about '70% acetic anhydride, the balance being water and acetic acid. However, the process may be applied equally well to other compositions, as,

for example, compositions containing between 40.

to 80% anhydride, the balance being water and acid.

The anhydride containing composition as ob# tained from the pyrolysis process was in the form of a cold liquid. In other words, the vapors containing anhydride are usually immediately condensed by refrigeration or other rapid cooling to form a cold liquid and thereby minimize the reaction between the acetic anhydride and any water present therewith.

Referring` to the drawing attached to the present application, column 2 has been started up by partially filling the column and base heater with acetic acid, supplying heat to the base heater to bring the column up to temperature, supplying cooling medium to the various ccndensers, and otherwise rendering the fractionation operative in the usual manner for carrying out distillations. The liquid mixture oi acetic anhydride, acetic acid, and water was fed through conduit 9-l0ii into column 2.

Acetic acid and Water vapor were volatilized from the top of the column through conduit li and were condensed in condenser E. Sufcient of the condensate was returned through conduit 8 to provide a reux ratio of approximately 3:1. It will be observed that this ratio is much lower than the ratio heretofore used and is entirely satisfactory because in column 2 no attempt is made to remove all extraneous components from the acetic anhydride. In other words, the fractionation carried out in col-umn 2 is preferably sufficient to remove a substantial part of the water from the anhydride so that anhydride collecting in base heater I4 still contains acetic acid but only a small amount, if any, of water.

From base heater I4 the vapors ofranhydride still containing acetic .acid pass up through con-y duit i5 into heat transfer device i2. inasmuch as these vapors are largely comprised of acetic anhydride, depending on the exact content thereof they would be at a temperature in excess of 120 C. or even 130 C. in some instances. Consequently, the cold raw feed entering through conduit 9 in the passage through conduit I 0 rapidly becomes preheated, condensing the anhydride-containing vapors to a hot liquid. This hot liquid flows through conduit 22 into the second fractionating column 3. It will Vbe observed that by the combination of columns as shown, heat economy may be effected since any heat not taken up by the incoming feed still remains in the hot liquid going to the vsecond column and .is utilized there.

Column 3 is operated generally .similar to column 2 excepting that due to the higher concentration of anhydride in the liquid being fractionated therein, base heater 23 may normally be carried at a somewhat higher temperature in order that acetic -acid having a boiling point of 118 C. is distilled out of the acetic anhydride. The acetic acid vapors together with any small amount of water fractionated in column 3 ows out through conduit I1 into condenser i8 wherein these vapors are condensed. Sufficient of :this acetic acid condensate is returned through conduit I9 to the head of the column for renuxing, the remainder of thecondensed acetic acid being drawn off through conduit 2I from which it may be returned to regular acid use or recycled `to acid pyrolysis for preparing further anhydride.

It will be observed that it is only necessary in the present invention, with the slightly longer columns already described, to redux through conduit I9 sunicient liquid to provide a redux ratio of about 3:1 as compared with the prior arts use of reflux ratios in the neighborhood of 8:1.

The vapors evolved from base heater 23 and Iiowing out through conduit 24 into condenser 25 are comprised of practically pure acetic anhydride which may be withdrawn at 25 and conducted to any of the desired uses for acetic anhydride.

By virtue of the particular construction shown for both base heaters lli and 23 applicants are able to accomplishthe circulation of the anhydride and acid as just described. That is, the upward velocity of the anhydride and acid Vto be heated up through the tubes exempliiied by tubes 35 has .backing it up the hydrostatic head of liquid in the down-comer exemplified by Dart 36 The down-comer permits the unvaporized portion of the base heater contents to return by way of the down-comer to the bottom of the base heater. However, at the same time there is provision for the disengagement of the vapors either through conduit i6 or 24 at the top of the down-comer from which the vapors of acid and anhydride or anhydride in the instance of conduit 24 may be removed for condensation. This circulating motion of the anhydride containing liquid in applicants process through the heating tubes scrubs the surface of the tubes and thereby discourages any solid materials from coating on the surfaces of the tubes. In other words, by this manner of circulation in applicants process the heat transfer is kept to a maximum because any fouling in the base heater at the principal point of heat transfer the tubes) is discouraged. Fouling in the lower part of the base heater away from the vicinity of heat transfer does not cut down on the efficiency of heating. The residue deposited in the lower part of the base heater may be removed in the usual manner by cleaning.

In accordance with this example, the feed mixture entering conduit 9 in addition to containing acetic anhydride, acetic acid, and water as above described also contained some butyric anhydride within the range of 4% to 16%. The process was operated as above described, and the butyric anhydride followed the acetic anhydride through the process. There was withdrawn at conduit 2'6 an anhydride mixture comprising greater than acetic anhydride but containing some butyric anhydride. This anhydride mixture was subjected to ordinary vacuum distillation for separating pure acetic anhydride from the butyric anhydride. Y In vanother example, it was noted that the feed entering conduit 9 contained 4% to 10% propionic anhydride. The processing was the same as in the instance of butyric anhydride and the acetic anhydride-propionic anhydride final product obtained through 2E was separated by conventional vacuum distillation. it has been observed that if the contents of other anhydrides suchv as butyric anhydride and ,propicnic an hydride constitute a very high percentage ofthe feed, it is preferred to dilute the feed with raw acetic anhydride so that the contents of the higher anhydrides donot exceed 25% or 30% and that the percentage of anhydride in the aggregate is preferably not more than 50% or 60%. The exact amount of .such dilution is not critical but preferably would be sufficient for decreasing the boiling point of the feed-liquidso that sufficiently high temperatures for distilling may be accomplished by steam heating.

The advantages of the method of the present invention wherein two fractionation columns are used in combination as above described, over. its use of two independent columns will be further apparent from the following table:

l PERFORMANCE OF REFINING STILLS Pounds of Steam Hookup Distillate Usage No. 1 Still Alone 33, U00 4 No. 2 Still Alone 35, C00 3 Nos. l and 2 Stills Comibned in A ordance with the Present Invention 166, 000 3 is somewhat more capital investment than with two independent columns in that there is the additional unit l2 and in that columns 2 and 3 each are a few plates longer, as already described, than when independent single columns are used.

However, all factors considered, it is believed that the double column combination exhibits substantial improvement in the magnitude oi the savings in heat input as well as the greater capacity of anhydride product handled and withdrawn from the double column combination.

While it is not desired to be bound by any theory of operation, it is believed apparent that one reason for the lower heat input required in the present invention is derived from the use of less reflux, above described, as for example, of only about 3:1 as compared with 8:1. Inasmuch as in the rst fractionation no attempt is being made to fractionate to 100% anhydride in one column the lower redux ratio is adequate. With two columns in combination, in accordance with the present invention as described, it is possible to operate the iirst column Where a lower boiling content is involved, at a more favorable temperature than Where concentrated anhydride having a higher boiling point is involved. Therefore, with the double column combination of the present invention better heating control is possible than with a single column.

It is, therefore, apparent from the foregoing that a new method and apparatus arrangement have been described for the separation and isolation of a relatively pure high grade anhydride from compositions wherein the anhydride is in the presence of acid, water, and other lower boiling components, the anhydride constituting only a portion of the composition. While in the present description the use of columns of appronimately the same size have been shown, the pres# ent invention is not limited in such details as it is possible to use columns of dissimilar size and make other reasonable changes in the present arrangement. In the event of using columns of dissimilar size, it would be preferred to place the larger column rst in order and throw a greater load thereon. Other changes will be apparent to those skilledinthe art. Y

iWe claim: i

1. A method of separating an anhydride of a loweralip'hatic acid from liquids containing the anhydride in the presence of lower aliphatic acid, water, and any other lower boiling constituents present which comprises feeding said liquid to a nrst distillation, heating the liquid in the lirst distillation in a heating zone connected for liquid passage through a downcomer flow Zone outside of the heating zone, which downcomer zone assists in disengaging vapors from liquids passing therethrough, subjecting the liquid in the first distillation to suflicient distilling and. frac tionation for removing at least a greater part of the water and some of the acid as head products of said first distillation, condensing at least a part of these head products' and returning condensate as reiiux to the rst distillation, accomplishing a preliminary separation of anhyride by'removing vapors principally comprised oi' the anhydride but still containing acid and some water from the portion oi said downcomer flow zone above the level of'liquid therein, pass'- ing these vapors in heat transfer relationship with said feed to the first distillation to condense the vapors to a hot liquid and preheat said feed by transfer of latent heat given up in the condensation of the vapors, flowing this hot liquid to a second distillation, heating this hot liquid in the second distillation suiiciently to distill ofi as head products substantially all of the acid and any water remaining in said anhydride, withdrawing and condensing these head products, returning a portion of the condensed head products as reflux, and volatilizing oli a substantially pure anhydride as the tail product of this second distillation.

2. A process for the separation of anhydrides from a mixture'of acetic anhydride, at least one aliphatic anhydride higher than acetic anhydride, acetic acid and water which comprises diluting the aforesaid anhydride mixture with raw acetic anhydride so that the content of the higher anhydride in the diluted mixture does not exceed 25 to 30%, and that the total percentage of anhydride is not more than 50-60% of the mixture being separated, subjecting the mixture thus diluted to a rst distillation wherein a substantial part of the Water and some of the acid is removed to give amore concentrated anhydride, transferring this more concentrated anhydride to a second distillation wherein acid and water remaining are distilled oil? leaving a substantially anhydrous mixture consisting essentially oi anhydrides and subjecting this mixture of anhydrides to vacuum distillation for separating the acetic anhydride from the higher anhydride.

DAVID C. HULL.

VILLIAM R. SAUNDERS.

References Cited in the file of this patent UNITED STATES PATENTS Y A Number Name Date 896,434 Chute Aug. 1S, 1993 1,507,108 Cyphers Sept. 2, 192e 1,940,699 Ricard et al Dec. 26, 1933 2,310,649 Peebles Feb. 9, 1943 2,504,195 Hall et al. Apr. 18, 1959 

1. A METHOD OF SEPARATING AN ANHYDRIDE OF A LOWER ALIPHATIC ACID FROM LIQUIDS CONTAINING THE ANHYDRIDE IN THE PRESENCE OF LOWER ALIPHATIC ACID, WATER, AND ANY OTHER LOWER BOILING CONSTITUENTS PRESENT WHICH COMPRISES FEEDING SAID LIQUID TO A FIRST DISTILLATION, HEATING THE LIQUID IN THE FIRST DISTILLATION IN A HEATING ZONE CONNECTED FOR LIQUID PASSAGE THROUGH A DOWNCOMER FLOW ZONE OUTSIDE OF THE HEATING ZONE, WHICH DOWNCOMER ZONE ASSISTS IN DISENGAGING VAPORS FROM LIQUIDS PASSING THERETHROUGH, SUBJECTING THE LIQUID IN THE FIRST DISTILLATION TO SUFFICIENT DISTILLING AND FRACTIONATING FOR REMOVING AT LEAST A GREATER PART OF THE WATER AND SOME OF THE ACID AS HEAD PRODUCTS OF SAID FIRST DISTILLATION, CONDENSING AT LEAST A PART OF THESE HEAD PRODUCTS AND RETURNING CONDENSATE AS REFLUX TO THE FIRST DISTILLATION, ACCOMPLISHING A PRELIMINARY SEPARATION OF ANHYDRIDE BY REMOVING VAPORS PRINCIPALLY COMPRISED OF THE ANHYDRIDE BUT STILL CONTAINING ACID AND SOME WATER FROM THE PORTION OF SAID DOWNCOMER FLOW ZONE ABOVE THE LEVEL OF LIQUID THEREIN, PASSING THESE VAPORS IN HEAT TRANSFER RELATIONSHIP WITH SAID FEED TO THE FIRST DISTILLATION TO CONDENSE THE VAPORS TO A HOT LIQUID AND PREHEAT SAID FEED BY TRANSFER OF LATENT HEAT GIVEN UP IN THE CONDENSATION OF THE VAPORS, FLOWING THIS HOT LIQUID TO A SECOND DISTILLATION, HEATING THIS HOT LIQUID IN THE SECOND DISTILLATION SUFFICIENTLY TO DISTILL OFF AS HEAD PRODUCTS SUBSTANTIALLY ALL OF THE ACID AND ANY WATER REMAINING IN SAID ANHYDRIDE, WITHDRAWING AND CONDENSING THESE HEAD PRODUCTS, RETURNING A PORTION OF THE CONDENSED HEAD PRODUCTS AS REFLUX, AND VOLATILIZING OFF A SUBSTANTIALLY PURE ANHYDRIDE AS THE TAIL PRODUCT OF THIS SECOND DISTILLATION.
 2. A PROCESS FOR THE SEPARATION OF ANHYDRIDES FROM A MIXTURE OF ACETIC ANHYDRIDE, AT LEAST ONE ALIPHATIC ANHYDRIDE HIGHER THAN ACETIC ANHYDRIDE, ACETIC ACID AND WATER WHICH COMPRISES DILUTING THE AFORESAID ANHYDRIDE MIXTURE WITH RAW ACETIC ANHYDRIDE SO THAT THE CONTENT OF THE HIGHER ANHYDRIDE IN THE DILUTED MIXTURE DOES NOT EXCEED 25 TO 30%, AND THAT THE TOTAL PERCENTAGE OF ANHYDRIDE IS NOT MORE THAN 50-60% OF THE MIXTURE BEING SEPARATED, SUBJECTING THE MIXTURE THUS DILUTED TO A FIRST DISTILLATION WHEREIN A SUBSTANTIAL PART OF THE WATER AND SOME OF THE ACID IS REMOVED TO GIVE A MORE CONCENTRATED ANHYDRIDE, TRANSFERRING THIS MORE CONCENTRATED ANHYDRIDE TO A SECOND DISTILLATION WHEREIN ACID AND WATER REMAINING ARE DISTILLED OFF LEAVING A SUBSTANTIALLY ANHYDROUS MIXTURE CONSISTING ESSENTIALLY OF ANHYDROUS AND SUBJECTING THIS MIXTURE OF ANHYDRIDES TO VACUUM DISTILLATION FOR SEPARATING THE ACETIC ANHYDRIDE FROM THE HIGHER ANHYDRIDE. 